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  • Geomagnetism, Rock Magnetism and Palaeomagnetism  (10)
  • Oxford University Press  (10)
  • 1
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    Critical single-domain grain sizes in elongated iron particles: implications for meteoritic and lunar magnetism (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-20
    Description: Kamacite particles (Fe–Ni, Ni 〈 5 per cent), are very common in extra-terrestrial materials, such as meteorites. It is normally assumed that for kamacite particles to be reliable recorders of magnetic fields, they need to be magnetically uniform (single domain, SD) and thermally stable. Larger particles subdivide into non-uniform multidomain (MD) magnetic structures that produce weaker magnetic signals, while small SD particles become magnetically unstable due to thermal fluctuations and exhibit superparamagnetic behaviour. In this paper we determine the first micromagnetic calculation of the stable SD range domain-state phase diagram for metallic iron; previous calculations were analytical. There is a significant increase in the critical size for the SD/MD threshold size, for example, for cube-shaped iron particles, the critical SD/MD threshold has now been estimated to be 25 nm, compared to 17 nm for previous estimates. The larger critical SD/MD threshold size for iron, agrees better with previously published nanometric observations of domain state for FeNi particles, then early analytical models.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 2
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    Magnetic properties as proxies for the evaluation of heavy metal contamination in urban street dusts of Nanjing, Southeast China (2014)
    Oxford University Press
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    Publication Date: 2014-10-08
    Description: Environmental magnetism can provide simple and fast methods to semi-quantify the contamination levels by heavy metals in urban areas from the relationships between the magnetic properties and the heavy metal concentrations. The aim of this study is to explore the interpretation of selected magnetic parameters as proxies for the evaluation of heavy metal contamination in urban dusts. Dust samples were collected from different districts of Nanjing, Southeast China. The magnetic properties of dusts were analysed and heavy metal (Fe, Al, As, Ba, Cd, Cr, Co, Cu, Hg, Mn, Mo, Ni, Pb, Sr, V and Zn) concentrations were measured. Magnetic minerals, mainly from anthropogenic activities, are dominated by coarse stable single domain-grained ferrimagnetic minerals. The – T curves indicated that the main magnetic minerals are magnetite and hematite. Magnetic properties exhibited similar spatial distribution and significant positive correlations with the concentrations of most metals and with the pollution load index. Magnetic properties had a strong link with the abundance of heavy metals derived from industrial emission and traffic activities, whereas there was a poor correlation with the concentrations of metals from multi-sources such as commercial/domestic sources and natural processes. Besides the low-frequency magnetic susceptibility ( LF ) and the saturation isothermal remanent magnetization, other magnetic parameters can be considered as efficient indicators in the assessment of heavy metal contamination. The linkage between the magnetic properties and heavy metal concentrations in street dusts depends on the sources and nature of their magnetic fraction. Environmental magnetism analyses may provide simple and rapid methods for the assessment of heavy metal contamination in urban street dusts.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 3
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    Variational data assimilation for a forced, inertia-free magnetohydrodynamic dynamo model (2014)
    Oxford University Press
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    Publication Date: 2014-10-17
    Description: Variational data assimilation (4DVar) is a powerful technique for tuning dynamic models to observations, in order not only to forecast future time evolution of the system, but to make inferences about quantities that are otherwise unconstrained by observation. We apply this technique, well-grounded in meteorology and oceanography, to the Earth's core where incompressible fluid motions in an electrically conducting medium are responsible for magnetic field generation. Our dynamic model's momentum equation neglects inertia such that the entire evolution depends only on the structure of the initial magnetic field; time evolution of the system is solely governed by the equation of magnetic induction. Nevertheless the dynamic system encompasses the effects of rotation, Lorentz forces and viscosity and aims to mimic a reasonable force-balance in the Earth's core. Building on the work of Li et al. , in order to optimize the data-fit subject to the dynamics, we further develop the mathematical structure of the adjoint equations of the system. We address the feasibility of recovering 3-D spatial properties of the system using only time-varying 2-D observations of different character. Using closed-loop testing, we demonstrate the retrievability of the initial state (and thus the entire trajectory) of the system over convective timescales, when sampling in regions in which magnetic induction dominates over diffusion. The results suggest the possibility of retrieving the entire trajectory of the dynamo system of the Earth using the 400-yr model of secular variation gufm1 .
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 4
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    Rock magnetic investigation of loess deposits in the Eastern Qingling Mountains (central China) and its implications for the environment of early humans (2016)
    Oxford University Press
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    Publication Date: 2016-09-21
    Description: The Luonan Basin, located in the transitional zone between temperate and subtropical China, is an important locality for human evolution during the early to middle Pleistocene. The loess deposits in the Luonan Basin contain numerous in situ lithic artefacts; the deposits also constitute suitable material for dating the artefacts and are potentially useful for reconstructing the climatic fluctuations which is important for studying the adaptation and occupation of the area by early humans. We carried out a combined rock magnetic and geochemical investigation of a loess sequence from the Liuwan Palaeolithic site in the Luonan Basin. The results indicate a mixture of magnetic minerals, including magnetite/maghemite and hematite/goethite. Magnetic susceptibility was used as a palaeoclimate proxy on the Chinese Loess Plateau; however, its application to the Luonan Basin may be problematic because the provenance of the loess parent material, as well as the depositional environment, differs from that of the Chinese Loess Plateau. We found that rock magnetic parameters related to the grain size of magnetic minerals, such as SIRM/ and ARM /SIRM, are better palaeoclimatic indicators than magnetic susceptibility. Overall, the magnetic results, together with the results of bulk grain-size and chemical index of alteration, indicate that the interglacial environment of early humans in Luonan Basin was warmer and more humid than the coeval environment of the Chinese Loess Plateau.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 5
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    Calibration of SQUID vector magnetometers in full tensor gradiometry systems (2014)
    Oxford University Press
    Details
    Publication Date: 2014-06-28
    Description: Measurement of magnetic vector or tensor quantities, namely of field or field gradient, delivers more details of the underlying geological setting in geomagnetic prospection than a scalar measurement of a single component or of the scalar total magnetic intensity. Currently, highest measurement resolutions are achievable with superconducting quantum interference device (SQUID)-based systems. Due to technological limitations, it is necessary to suppress the parasitic magnetic field response from the SQUID gradiometer signals, which are a superposition of one tensor component and all three orthogonal magnetic field components. This in turn requires an accurate estimation of the local magnetic field. Such a measurement can itself be achieved via three additional orthogonal SQUID reference magnetometers. It is the calibration of such a SQUID reference vector magnetometer system that is the subject of this paper. A number of vector magnetometer calibration methods are described in the literature. We present two methods that we have implemented and compared, for their suitability of rapid data processing and integration into a full tensor magnetic gradiometry, SQUID-based, system. We conclude that the calibration routines must necessarily model fabrication misalignments, field offset and scale factors, and include comparison with a reference magnetic field. In order to enable fast processing on site, the software must be able to function as a stand-alone toolbox.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 6
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    A 3-D conductivity model of the Australian continent using observatory and magnetometer array data (2014)
    Oxford University Press
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    Publication Date: 2014-06-30
    Description: An exploratory 3-D model of the electrical conductivity structure of the Australian continent is presented. The model is derived from the inversion of vertical magnetic-field transfer functions from the Australia-wide Array of Geomagnetic Stations. Crustal conductivity anomalies evident in the model are consistent with those previously mapped by independent magnetometer array studies and new electrical structures are suggested in the upper mantle. The model represents a seamless continent-scale basis for further models likely to be derived from subsequent studies. The model reveals three upper-mantle enhanced-conductivity anomalies beneath Archaean cratonic regions and two upper-mantle anomalies beneath Phanerozoic terranes in eastern Australia. Two of these anomalies have been investigated by recent magnetotelluric (MT) surveys, one in the Yilgarn Craton–Officer Basin–Musgrave Block the other in the Gawler Craton region, and are consistent with the MT results. Across much of central Australia enhanced conductivity at depths of 50–100 km is observed in the model. This region corresponds well with a recognized seismic velocity gradient at 75–100 km. Conductivity differences are also observed beneath Archaean cratons in Western Australia. The Pilbara Craton is represented as an enhanced conductivity anomaly at about 100 km, corresponding well with the lower-velocity anomaly evident in surface wave tomography models. The Yilgarn Craton is imaged as a low-conductivity body, with conductivity two orders of magnitude lower than the Pilbara Craton, continuing to greater depths.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 7
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    Efficiency of stepwise magnetic-chemical site assessment for fly ash derived heavy metal pollution (2015)
    Oxford University Press
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    Publication Date: 2015-09-08
    Description: Previous works revealed a close relationship between magnetic susceptibility (MS) and heavy metal (HM) contents originating from industrial sources. However, despite general statements on the usefulness of magnetic mapping, the benefit of this procedure for geochemistry was not quantified yet. We present a study on fly ash pollution in soil around a coal-burning power plant complex and simulate a stepwise approach of magnetic pre-screening and subsequent targeted sampling for chemical analysis. The aim of this study is not to discuss correlations between MS and HM, but to show that a combined stepwise magnetic-chemical approach is the most efficient way for outlining HM contamination. In order to provide quantitative evidence, we explored map similarities of spatial HM distributions based on magnetochemical data and chemical data only. We determined 3-D triangular planes defined by categorized HM values at the sampling coordinates and calculated the average dihedral angle of the normal vectors as a similarity result. The study shows that the ‘Targeted’ HM map (selection of 30 sites based magnetic pre-screening) has a higher similarity with the ‘True’ Pollution HM map (85 sites) than HM maps resulting from site selections (30 sites) without using magnetic pre-screening information.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 8
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    New archaeomagnetic direction results from China and their constraints on palaeosecular variation of the geomagnetic field in Eastern Asia (2016)
    Oxford University Press
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    Publication Date: 2016-10-09
    Description: We carried out an archaeomagnetic directional study on 38 oriented samples (bricks and baked clays) collected from four archaeological locations at three provinces in China. The ages of our samples, spanning from ~3000 BCE to ~1300 CE, were constrained using a combination of archaeological context, radiocarbon dating and stratigraphic information. Rock magnetic results demonstrate that the main magnetic minerals of the studied samples are magnetite and/or hematite in single domain and superparamagnetic states. A total of 20 new reliable archaeodirectional data from 12 independent sites are obtained after thermal demagnetization experiments. These are the first set of archaeodirectional data in China produced since the 1990s. The published data are largely from the past 2 kyr and data from older time periods are rare. Our new data, especially those from period older than 3 ka, fill many gaps of the presently published dataset and will provide strong constraints on palaeosecular variation of the geomagnetic field in Eastern Asia and on the improvement of global models. Quite a few inflection points in the direction of the geomagnetic field are recorded in Eastern Asia over the past 10 kyr and some of them synchronize with the maximums or minimums of the palaeointensity. The palaeosecular variation rates are very low (based on present data distribution) before 2000 BCE and then start to increase and fluctuate afterward, which is generally consistent with the pattern of palaeointensity variations in this area.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 9
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    Magnetic properties of Archean gneisses from the northeastern North China Craton: the relationship between magnetism and metamorphic grade in the deep continental crust (2015)
    Oxford University Press
    Details
    Publication Date: 2015-03-05
    Description: Magnetic mineralogy of crustal rocks has important implications for understanding continental crustal evolution and origin of regional magnetic anomalies. However, magnetic properties of the deep continental crust are still poorly understood. In this paper, measurements of density ( ), mass-specific magnetic susceptibility ( ), natural remanent magnetization (NRM) and magnetic hysteresis loops, temperature-dependent magnetic susceptibility ( – T ), chemical and mineral analyses were conducted on Archean gneiss samples from the Jixian petrophysical section in the Precambrian terrain, northeastern North China Craton, with the aim of refining understanding of magnetic phase transformations in the deep crustal rocks. Results show that density and rock magnetic properties change distinctly with metamorphic facies. The dominant magnetic mineral is magnetite, while a little hematite is present in a few samples. Together with geochemical and mineralogical compositions, it is inferred that progressive increase in metamorphic grade from east to west is the major cause for magnetic enhancement of the lower crust in the studied section. Therefore, we conclude that study of magnetic phases of deep crustal rocks can offer important insights into the history of high metamorphic grade terranes.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 10
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    Magnetic properties of Archean gneisses from the northeastern North China Craton: the relationship between magnetism and metamorphic grade in the deep continental crust (2015)
    Oxford University Press
    Details
    Publication Date: 2015-03-04
    Description: Magnetic mineralogy of crustal rocks has important implications for understanding continental crustal evolution and origin of regional magnetic anomalies. However, magnetic properties of the deep continental crust are still poorly understood. In this paper, measurements of density ( ), mass-specific magnetic susceptibility ( ), natural remanent magnetization (NRM) and magnetic hysteresis loops, temperature-dependent magnetic susceptibility ( – T ), chemical and mineral analyses were conducted on Archean gneiss samples from the Jixian petrophysical section in the Precambrian terrain, northeastern North China Craton, with the aim of refining understanding of magnetic phase transformations in the deep crustal rocks. Results show that density and rock magnetic properties change distinctly with metamorphic facies. The dominant magnetic mineral is magnetite, while a little hematite is present in a few samples. Together with geochemical and mineralogical compositions, it is inferred that progressive increase in metamorphic grade from east to west is the major cause for magnetic enhancement of the lower crust in the studied section. Therefore, we conclude that study of magnetic phases of deep crustal rocks can offer important insights into the history of high metamorphic grade terranes.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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